I would say 'very'. I have three and they made it through a Colorado winter just fine. Now, we did bring them in when the temperature dropped too much all at once and had a warming pad out there (they loved it) but they probably would have been just fine even without the fuss. They hated snow though. There were days they refused to come out or only came out for a minute or two because they couldn't stand walking in snow.
Edit to add: I should note that they were only 18ish weeks when Winter officially hit.
We have one and she does well even when it's really really cold. We didn't have a heater last winter, and I think our Austrolorpe got a few spots of frostbite on her comb, but with the ee's little pea comb she did better than those with big combs.
I have EE's and last years Indiana winter was really cold. I have an insulated coop and one heat light to warm about 8x10 area. They did really well. Some of my dual purpose breeds didnt fair as well. I also use a deep litter during winter. I dont let them out if I walk outside and its "freeze your nose hairs" cold lol. I dont know what temperature that really is but if I cant breathe well then they cant either imo.
EE's are great birds! I live in MN and they do just fine. They are very hardy and you don't have to worry about their tiny pea combs. I do not heat my coop but do have a 75 watt light bulb in an aluminum brooder hood. Funny though, they don't go under it very much even when it is bitter cold. (like minus degrees for weeks in a row).